A mobile communication system generally refers to any telecommunication system which enables wireless communication when a user is located within the service area of the system. Examples of such systems are cellular mobile communication systems, such as GSM (Global System for Mobile communications), or corresponding systems, such as PCS (Personal Communication System) or DCS 1800 (Digital Cellular System for 1800 MHz), third-generation mobile systems, such as UMTS (Universal Mobile Communication System) and systems based on the above-mentioned systems, such as GSM 2+ systems and the future fourth-generation systems. One typical example of a mobile communication system is the public land mobile network PLMN.
The invention and its background will be discussed below by using a GSM 2+-system called GERAN (GSM/EDGE (Enhanced Data rates for Global Evolution) Radio Access Network) Release 5 Iu as an example yet without limiting the invention thereto. ‘Iu’ means that mobile stations are connected to a radio access network GERAN that is further connected with Iu interfaces to the core network providing the data transfer.
The protocol architecture of the air interface of GERAN Iu, called Um interface, comprises three protocol layers: a physical layer L1, a data link layer L2 and a network layer L3. The data link layer L2 of GERAN Iu comprises a radio link control RLC sub-layer and a medium access control MAC sub-layer, which are common for a user plane (i.e. for user data) and control plane (i.e. for signaling data). The layers above RLC are PDCP (Packet Data Convergence Protocol) for the user plane and RRC (Radio Resource Control) for the control plane. RLC provides reliable mechanisms, such as an acknowledged mode ARQ for transmission of upper layer data over the air interface.
Each radio bearer has an RLC instance transmitting the radio bearer data for peer-to-peer information change. The RLC instance transmits information by means of data blocks called packet data units over the air interface on an L2 link established for a radio bearer. In the acknowledged mode and in an unacknowledged mode, the L2 link is called a Temporary Block Flow TBF in GERAN. The L2 link, hereinafter called TBF, is a carrier (i.e. allocated radio resource) that supports the unidirectional transfer of packet data units. Each packet data unit PDU originates from a certain RLC instance. At a transmitting site, the RLC instance (an RLC transmitter) forms RLC packet data units by segmenting the upper layer data into PDUs to which layer 2 control information is added. Each PDU is independently protected against degradation caused by the radio channel. At a receiving site, the RLC instance (an RLC receiver) re-assemblies the RLC packet data units into upper layer data.
In the acknowledged mode, a mechanism called a sliding window is used to control the flow of RLC packet data units across TBF in the transmitter. As each packet data unit is transmitted, an upper window edge UWE is incremented by unity. Similarly, as each packet data unit is acknowledged, a lower window edge LWE is incremented by unity/acknowledged packet data unit. The sending of new packet data units is stopped, when the difference between UWE and LWE becomes equal to the size of the RLC send window. The situation is called window stalling. The RLC window size represents the size of an RLC memory reserved for an RLC instance and it should be big enough so that resources allocated to TBF can be benefited. A prior art mobile station supporting multislot capability, i.e. a mobile station capable of using more than one timeslot for one TBF, must support the maximum RLC window size corresponding to its multislot capability. For example, if the mobile station is capable of three timeslots on a downlink and one timeslot on an uplink, the mobile station has to support an RLC window size of 384 in the downlink and 192 in the uplink direction.
The mobile stations according to the GERAN Release 5 Iu may support multiple TBFs allowing several RLC instances run in parallel. Each RLC instance will have an RLC window, i.e. there are as many parallel RLC windows as there are RLC instances running in parallel. If the RLC window sizes are defined according to the prior art, the mobile station should support, for each RLC instance that may run in parallel, the maximum window size corresponding to the number of timeslots the mobile station can at most use for TBF transferring packet data units from the RLC instance. For example, if two RLC instances may run in parallel, both of them being able to use a two-timeslot TBF, the mobile station should support two parallel RLC windows both having an RLC window size of 256. This can also be expressed in another way: if the mobile supports two timeslots and has an RLC memory, the size of which is 512, only two RLC instances are allowed to run in parallel. However, the network may allocate a smaller window size in order to optimize the number of users of the air interface or the memory consumption of the network, for example. Thus, the network may allocate only one timeslot for each TBF of the previous example, the timeslots corresponding to a window size of 64 causing a memory consumption problem: only 25 percent of the memory reserved in the mobile station for the windows would actually be used. The problem can also be seen as unnecessarily limiting the amount of parallel RLC instances: only two RLC instances in parallel are allowed although eight RLC instances with window sizes of 64 could run in parallel. Furthermore, the memory resources in a mobile station are much more limited than in the network, and therefore reserving memory resources in such a manner that for each allowed parallel TBF, a memory resource corresponding to the maximum window size for TBF can be allocated simultaneously, is a waste of limited memory resources and limits unnecessarily the amount of parallel RLC instances. The problem is emphasized when the mobile station is at a receiving site, since the receiver only buffers the packet data units which are correctly received and are waiting for reassembly and transmission to the upper layer. Usually the buffered packet data units require a great deal less memory than the corresponding maximum window size.